SOIL WATER DYNAMICS IN NATURAL PINE FOREST AND DISTURBED GRASSLAND

Soil water dynamics were investigated under two different types of vegetation sites at the Environmental Research Center, University of Tsukuba, Japan, a disturbed grassland and an adjacent pine forest. The evaporation effective zones and soil water dynamics were determined by evaluating the soil physical properties, pressure head (for every 30 minutes), and isotope analysis (d ¹⁸ O) of soil water for the two sites. The results showed highly different physical properties for the two sites. At the pine forest site, a total porosity from the ground surface to the depth of 0.7 m was nearly constant, that is, between 81% and 84%, and then decreased uniformly with the depth to reach 63.2% at 1.5 m. At the grassland site, the total porosity showed more complex pattern. It was about 70% near the ground surface, then expeditiously decreased to approximately 62% between the depths of 0.1 and 0.4 m, then increased to the maximum of about 77% between the depths of 0.5 and 0.8 m, and then decreased again to 54.9% at the depth of 1.5 m. The total pressure head indicated that the evapotranspiration effective zone of the pine forest and the grassland was 0.7 and 0.5 m, respectively. In association with the different physical properties, the dynamics of the soil water also varied with the two vegetation covers. Consequently, the evaporation and transpiration effective zones were also different during the cyclic processes. The soil water characteristic curves clearly showed that the pine forest's soil water was movable at all depths (0 - 1.0 m). The isotopic ratio of (d ¹⁸ O) of soil water indicated, that the evaporation effective zone in the pine forest was 0.3 m while in the grassland it was 0.4 m. According to the total head profiles the evapotranspiration effective zone in the pine forest was 0.7 m, thus the excess of 0.4 m is considered to be due to the transpiration. In the grassland, the respective difference was only 0.1 m.